This invention relates to integrated circuit (IC) packages. More particularly, this invention relates to leadframes and methods for manufacturing leadframes for use in IC packages.
In normal operation, IC dies produce heat which must be efficiently dissipated to prevent damage to the dies. Heat dissipation is a particularly acute problem for high power devices.
One common technique for effectively dissipating heat from an IC package involves placing a heat sink on top of the IC package, opposite to the side of the package that is mounted to a printed circuit board. Heat sinks, however, are generally large and are therefore not optimal or practical in many instances, such as in mobile electronic devices, where the size of the IC packages must be small to fit within the confines of the device (e.g., a cell phone).
One conventional small form factor package uses the printed circuit board upon which the package is mounted, in effect, as a heat spreader. According to this design, the bottom of a die paddle, upon which the die is mounted, is exposed so that it is in direct contact with the printed circuit board. This design effectively transfers heat to the printed circuit board.
FIG. 1 is a cross-sectional side view of a known IC package in which the die paddle (upon which the non-active side of a die is disposed) is configured to have a surface that forms a portion of one of the exterior surfaces of the package. In this manner, the exposed side of the die paddle may easily be placed in contact with another surface to dissipate heat from that is thermally transferred from the die.
As shown in FIG. 1, an IC die 10 is disposed on a die paddle 12, which is supported by a number of support members 18 connected to a frame 14. Die paddle 12, frame 14 and support members 18 together form a leadframe. A number of leads (not shown) extend from the package to provide the electrical connections from die 10 to external circuitry. The leads are electrically connected to the die by bond wires 13, which are individually connected from one of a number of bond pads (not shown) disposed on die 10 to the leads. Once all of the bond wires 13 have been connected, molding compound 16 surrounds die 10, die paddle 12 (except for the bottom exterior surface portion described above) and frame 14.
In the package shown in FIG. 1, the bottom of die paddle 12 is aligned on a plane that is parallel to, but lower than, the plane on which the leads of frame 14 are aligned. To support die paddle 12 in its lower position, support members 18 are connected between die paddle 12 and frame 14.
There are many known techniques for lowering die paddle 12 so that its upper surface is substantially parallel to frame 14. One of those techniques involves bending support members 18 down so that they form an angle between the upper and lower planes. This technique is illustrated in FIGS. 2A and 2B (and may, for example, result in a leadframe being formed similar to that shown in FIG. 1).
FIG. 2A is a cross-sectional side view of die paddle 12, frame 14 and support members 18 prior to the bending operation. Connection points 19 represent the place where die paddle 12 and support members 18 are connected together (and one of the locations at which bending will occur). As shown in FIG. 2A, die paddle 12 and support members 18 are initially co-planar (as is frame 14).
FIG. 2B is an overhead view of the leadframe assembly shown in FIG. 2A. In addition to die paddle 12 and support members 18, FIG. 2B also shows the configuration of frame 14, which surrounds die paddle 12 and support members 18. Moreover, FIG. 2B shows that, in this instance, there are four support members 18, each of which is curved to accommodate the bending operation as follows. To vertically offset die paddle 12 into the lower plane, die paddle 12 is gently pushed down, which causes support members 18 to bend and straighten out. If die paddle 12 is lowered a maximum distance, support members 18 will be substantially straight, as is illustrated by the leadframe shown in FIG. 1.
As illustrated in FIG. 1, the offset angle between one of the support members 18 and a vertical axis (as shown by the dashed line in FIG. 1), as projected on to a vertical plane may be, for example, forty-five degrees (45xc2x0) or more. The offset angle results in a lengthwise offset xe2x80x9cLxe2x80x9d (which is labeled xe2x80x9cOFFSETxe2x80x9d in FIG. 1) between die paddle 12 and frame 14. Large offset angles using known bending techniques often result in an OFFSET of five mils or more.
Large offset angles, which result in large OFFSETs, are generally undesirable because they necessarily decrease the allowable area for the die. In particular, a die paddle""s length (and therefore the permissible area for the die) is limited because the ends of the die paddle are offset from the frame by a length of twice L.
In addition to limiting the die surface area, package configurations such as those shown in FIGS. 1, 2A and 2B also may tend to limit the possible numbers of electrical connections between a die and a leadframe. Specifically, referring again to FIG. 1, in known packages, a bond wire 13 runs from the die 10 and connects to one of the support members 18 adjacent to the die paddle 12, on the horizontal plane along which the die paddle 12 lies. The bond wire 13 typically serves as a low impedance ground connection between the die 10 and the frame 14. In other words, the frame 14 serves as a ground to the die 10. Although it is often desirable to have such a ground connection, the existence of the additional bond wire 13 increases the complexity of the package, which in turn makes the package harder to design. This difficulty is exacerbated by limiting the location of the connection between a die and a support member to the lower horizontal plane.
Therefore, it would be desirable to provide leadframes in which the permissible area for the die was increased in comparison with known techniques.
It would also be desirable to provide leadframes in which the offset between the die paddle and the frame were reduced.
It would be further desirable to provide leadframes that allow greater design flexibility with regard to the position of bond wires that serve to connect a die with a ground or a signal that is passed through the body of a leadframe.
In accordance with these and other objects of the present invention, novel lead frame geometries and methods of making the same are presented in which, for example, the permissible area for the die paddle is increased. The advantages of these geometries include, but are not limited to, decreasing the offset of a die paddle.
According to an embodiment of the present invention, a leadframe has a frame, a die paddle with a receiving surface, and a number of support members that connect the frame to the die paddle. As is described more fully below, the support members extend in a direction that is substantially parallel to the side of the die paddle to which they are attached, rather than being perpendicular to that surface. Because the support members extend in a direction that is mainly parallel to the side of the die paddle to which they are attached, as opposed to extending directly towards that side of the die paddle, the amount of room available to the die paddle is increased. Thus, a leadframe constructed in accordance with the present invention may be used with larger die paddles (and accordingly, larger dies), as compared to leadframes in which the support members extend substantially perpendicular to the side of the die paddle to which they are attached.
Moreover, the leadframe further has a number of leads connected to the frame that will eventually serve to electrically connect an integrated circuit (i.e., the die) mounted on the die paddle to an external electrical device.
The frame, which lies in an upper horizontal plane, generally has the shape of a rectangle that defines an interior portion. Along with the frame, the leads lie along the upper horizontal plane. Each of the leads has an outer end attached to the frame and an inner end within the interior portion.
The die paddle lies in a lower horizontal plane that is wholly within the confines of the exterior surfaces of the frame. A number of support members connect the frame with the die paddle. More particularly, each of the support members has a proximal end that is connected to the frame at a corresponding frame attachment point and a distal end connected to the die paddle at a corresponding die paddle attachment point, proximate to a corner of the die paddle. The frame has a number of extension members that extend from a side of the leadframe in a direction perpendicular to that side. Each of the proximal ends of the support members is attached to a corresponding one of these extension members.
Projected on to a first vertical plane that is perpendicular to the upper and lower horizontal planes and that is perpendicular to the side of the frame to which the corresponding support member is attached, is an offset angle between a support member and a vertical axis that is less than 45 degrees. According to the present invention, it is possible to make the offset angle 0 or even negative. By enabling the leadframe to have offset angles less than 45 degrees, leadframes constructed in accordance with the present invention may have die paddles that are larger than those in previously known leadframes, which in turn enables the leadframes to be used with larger dies. The present invention also provides other advantages, however, such as allowing electrical interconnections between a support member and a die, even in cases where the leadframe is designed with a relatively large offset angle.
Projected on to a second vertical plane that is parallel to the side of the frame to which a support member is attached, the support member extends at 45 degree angles (xe2x80x9cspread anglesxe2x80x9d) from the vertical axis.
In an alternate embodiment, a die paddle and frame are disposed along lower and upper horizontal planes as above, and each of a pair of support members is connected to a frame by a stem that has both horizontal and vertical offsets from the frame. Each of the support members is disposed relative to the stem in the same manner the support members are disposed relative to the frame in the originally described embodiment.
In yet another alternate embodiment, a bond wire connects a die directly to the body of a frame at some point on the upper horizontal plane. Direct die to frame connections typically serve to provide the die with a low impedance connection to ground. However, direct die to frame connections could serve other purposes. Allowing die to bond wire connections to be made on the upper horizontal plane increases design flexibility.